The trend can be interpreted in so many different ways, and most of them sound very plausible.
1-Post-timeskip arcs are not as exciting to the audience as the pre-timeskip ones. I don't think this is the case, but further search is necessary.
2-People dislike the separation of the strawhats, some will only buy volumes that has their favorite one. Again, this highly unlikely to be the case, as Luffy, Nami, Sanji, Chopper, and Jinbe were all in the top 10 in the latest poll, and are all present in the current arc. Furthermore, one of the most popular arcs, the battle of marineford, only had Luffy. But still, having the full crew in one place might increase the sales.
3-Marketing problems. I don't know if this is really the case, specially with the spectacular 20th anniversary campaign, but maybe they should've adverised the current arc instead of Wano. Maybe adveritse relevant characters, arcs, and moments, instead of showing "I wanna live" in game trailers, maybe they should show "Raizou is safe" or "Without you I can't become the pirate king" moments.
4-The stagnation of the anime. I mean seriously, even with the extra care put in the recent anime episodes, one can't deny that the anime is in a bad state. With a bad timeslot, an uneven airing of episodes across the country, and a lack of filler arcs, the anime has all the ingredients to be less than optimal. Perhaps they should adapt the ace novels, the omakes, andthe cover stories, at least make it something other than "One Piece manga the anime".
5-Franchise fatigue. This is self explanatory. People are bound to get tired from reading a series for 20 years straight. Thankfully, the solution here is simple, just let people rest for a while, stop making hugely advertised big movies, specials, and games, and then boom, revive the hype for the next important arc.
6-Weekly Shonen Jump decline. Yes, WSJ is currently facing a slow but apparent decline. A strong compitetor in WSJ is important because it drives people to buy new issues, and inderectly causes more people to get exposed to the new chapter of One Piece, this facilitates new discussions regarding the new chapters, which can cause more people to buy the volumes.
7-The manga industry itself is facing problems. With the increased, and still increasing accessibility of the internet, pirating the chapters is becoming more and more common. The industry itself started taking measures. Another problem could be that with many manga out there, people are getting exhausted . There's nothing that can be done about that though.
8-The decreasing birthrate. Sadly, this dilemma can only be solved by the government.
Thankfully, the trend isn't bothering me all that much, because I believe it'll end with the upcoming olympics. And even if it didn't, the masses that bought all 60 volumes of One Piece will certainly buy the rest, next year, the year after, or maybe after the manga ends. Yet, the idea that potential fans are being pushed away stings me as well.
–- Update From New Post Merge ---
Thanks sandman for the translation!!!
Guys, remember that Oricon Manga sales count only paper version of Manga Volumes. Digital sales are not included.
Digital sales have been growing year by year in the last 3-4 years in Japan. It's normal that physical sales had decrease in last years becouse of digital sales.
16 characters of totally agree